Great strides have been made in the battery industry for improving the reliability and lifetime of DC voltage supplies Alkaline and lithium batteries can now provide useful lifetimes much longer than dry cell batteries heretofore known.
Because of the advances in the battery field, electrical apparatus has become more dependent on batteries for supplying the voltage thereto While batteries present an economical and reliable source of voltage for many types of electrical apparatus and circuits, one shortcoming is that the life of a battery is difficult to determine It is expedient in some situations to simply operate the electrical apparatus until the batteries are dead, while in other situations the batteries may be changed or recharged on a regular schedule to assure reliable and continued operation. While this last approach is very effective in terms of reliability, the batteries are generally replaced while there is yet some useful life available.
Some applications, especially in the medical field, require the reliable operation of battery-powered circuits, and thus are monitored accordingly. Other applications, such as watches, cameras, etc., preferably include monitor circuits for providing the user with a visual indication of a low voltage condition. U.S. Pat. No. 4,463,271 is exemplary of such a battery monitor circuit. The circuit of the noted patent includes a number of comparators, a reference voltage and a current source. The number of circuit components thus required is considerable, even for a single supply monitor, thus making it less attractive for integration with other circuits.
It can be seen from the foregoing that a need exists for a simple uncomplicated voltage monitor circuit requiring few components, and thus being easily integrated with other circuits An associated need exists for a voltage monitor for detecting when the outputs of a dual series supply circuit decays asymmetrically. Yet another need exists for a technique to enable the adjustment of the monitor point during post-fabrication processing of the integrated circuit chip.